Ureas isocyanate reactions

Industrially, polyurethane flexible foam manufacturers combine a version of the carbamate-forming reaction and the amine—isocyanate reaction to provide both density reduction and elastic modulus increases. The overall scheme involves the reaction of one mole of water with one mole of isocyanate to produce a carbamic acid intermediate. The carbamic acid intermediate spontaneously loses carbon dioxide to yield a primary amine which reacts with a second mole of isocyanate to yield a substituted urea. 

Primary cycloaUphatic amines react with phosgene to form isocyanates. Reaction of isocyanates with primary and secondary amines forms ureas. Dehydration of ureas or dehydrosulfuri2ation of thioureas results in carhodiimides. The nucleophilicity that deterrnines rapid amine reactivity with acid chlorides and isocyanates also promotes epoxide ring opening to form hydroxyalkyl- and dihydroxyalkylaniines. Michael addition to acrylonitrile yields stable cyanoethylcycloalkylarnines. 

Isocyanates react with carboxylic acids to form amides, ureas, anhydrides, and carbon dioxide, depending on reaction conditions and the structure of the starting materials (Scheme 4.13). Aliphatic isocyanates more readily give amides. Aromatic isocyanates tend to react with carboxylic acids to first generate anhydrides and ureas, which at elevated temperatures (ca. 160°C) may further react to give amides. In practice, the isocyanate reaction with carboxylic acid is rarely utilized deliberately but can be an unwanted side reaction resulting from residual C02H functionality in polyester polyols. 

The stability of o-sulfonylbenzonitrile oxides and their thiophene analogs probably depends on electronic factors. The same factors do not prevent dimerization, as can be seen from data concerning several differently substituted nitrile oxides of the thiophene series (103). Sterically stabilized 3-thiophenecarbonitrile oxides 18 (R = R1 = R2 = Me R = R2 = Me, R1 = i -Pr), when boiled in benzene or toluene, isomerized to isocyanates (isolated as ureas on reaction with aniline) while nitrile oxides 18 with electron-withdrawing substituents (R1 and/or R2 = SOiMe, Br) dimerized to form furoxans 19. 

The reviewer did the reaction on loose resin (because MicroTubes are no longer available) and washed it extensively in the isocyanate reaction with DMF to remove the unwanted symmetrical urea. 

For this library, we chose to use three types of isocyanates (neutral, electron rich, and electron deficient) to demonstrate the broad utility of the urea-formation reactions. Employing the above strategy and using the split-and-pool approach, we synthesized a 27-membered urea library with purities ranging from 95 to 99%. All the compounds prepared were characterized by 1FI NMR and mass spectroscopy. Acetonitrile can also be used as a substitute for DCM, but lower yields and product purities are generally observed. Attempts to use other protic solvents, such as isopropyl and ethyl alcohol, were unsuccessful. The best results were achieved when a chlorinated solvent (DCM) was used. The structure identity of all products was confirmed by 1FI NMR and MS spectroscopy. Expected molecular ions (M + Na+) were observed for all the products, and in all cases as the base peak. The compounds and yields are listed in Appendix 3.1. 

It should be noted that isocyanates react with DMF to form bicychc ureas or isocyanurates (Chadwick and Cleveland, 1981). DMF is an unsuitable solvent for wood isocyanate reactions because of these side reactions. 

A methyl isocyanate-functionalized resin 8 has been used to sequester excesses of amines or hydrazines from solution phase when these reactants were used in urea-forming reactions or pyrazole-forming reactions.19,21 Finally, the a-bromoketone resin 9 was shown to efficiently sequester thioureas from solution in Hantzsch aminothiazole-forming reactions.27... 

Catalysis by cobalt(III) has been the subject of several papers.185-187 The N.N-bis(sahcyldene)ethylnediaminocobalt(III)-catalysed oxidative carbonylation of o-, m-and -substituted primary aromatic amines in MeOH gives ureas, isocyanates, carbamates, and azo derivatives. A Hammett p value of —0.5 for the reaction indicates that electrophilic attack of CO at a nitrogen anion complexed to Co in the TS is... 

The isocyanate group is very reactive and will condense with other active compounds containing hydrogen, like amines and thiols. Products resulting from such reactions, respectively, are the ureas and thiocarbamates. With amines the rate of reaction is somewhat faster than with hydroxyl compounds, while that with thiols is slower. However, a wide variety of catalyst systems (many based on amines and organometallic compounds) is available for the acceleration of isocyanate reactions. 

In the case of softer electrophiles, such as methyl isocyanate, reaction is on the exocyclic nitrogen atom and the resultant urea (12) is stable and does not undergo Dimroth rearrangement (Scheme 4) (81JPR279). 

These herbicides contain nitrogen and respond to both FID and AFID. However, the analysis of these compounds is complicated by their thermal instability. For example, the total ion current trace obtained from the GC-MS ofamethanolic solution of monolinuron shows four peaks (Fig. 9). The formation of these peaks is illustrated in Fig. 10. This thermal decomposition appears to be general for the substituted ureas and reaction of substituted phenyl isocyanates with higher alcohols also occurs. As certain substituted alkyl phenylcarbamates are used as pesticides in their own right, inert solvents such as ethyl acetate or hexane should always be used for injection. The mass spectra of the thermal... 

It was also shown that adding 0.025 M AT-phenyl-AT -o-tolyl urea to the solution of phenyl isocyanate and o-toluidine catalysed the reaction so that the initial rate was as great as the ultimate rate in the absence of added urea. The reaction sequence proposed as a suitable explanation of these observations [178, 179] was... 

There are a few other chemical reactions on the wood surface that could make important contributions. One is that of moisture on the surface of wood to form an unstable carbamic acid group that quickly decomposes to form a primary amine with evolution of carbon dioxide. The primary amine formed has active hydrogens reactive to isocyanate. Other successive reactions ensue leading first to disub-stituted ureas and then to biurets. Furthermore, isocyanate reaction with urethane to form allophanates, and trimerization of isocyanates to form isocyanurate are also possible to variable extents, under the conditions of bonding. The different reactions are summarized in Scheme 2. 

A more detailed quantitative study of the water-o-tolyl isocyanate reaction by Shkapenko et al. 51) showed that at 80° in dioxane solution and in the presence of triethylamine or other catalysts the consumption of the isocyanate was complete within a short period when only approximately half of the theoretical amount of carbon dioxide was released. The evolution of carbon dioxide proceeded from this point on at a slow rate. It was also demonstrated that by heating the reaction mixture to 100°, 30-35% of theoretical CO2 was released, and that this portion of the CO2 was given off by the decomposition of the carbamic acid anhydride formed from the acid and a second molecule of isocyanate. Additional tests showed that 4-5% of the isocyanate formed o-tolyl ammonium-W-o-tolyl carbamate, 18.7% of the water added remained unreacted, and that a trace of the free o-tolyl amine was also present. In addition, the presence of di-o-tolyl urea was proven. 

Since according to these results 18% of water remained unreacted at a time when all of the isocyanate had been consumed, it is apparent that the isocyanate must have reacted with the diphenyl urea—a reaction that was actually demonstrated by these authors to occur at 80° under the experi-... 

In this section isocyanate-based polyurea foams ja-oduced by the reaction of the water-isocyanate or amine-isocyanate reactions will be described. Urea-formaldehyde foams will be excluded. The isocyanate-based urea-linkage formation is shown by the following model reactions ... 

Some examples of urea foams prepared by the water-isocyanate reaction are as follows. ICI disclosed the foam prepared by the water-isocyanate reaction in the jn-esence of imidazole compounds (137). PRB NV disclosed a foam j epared in the presence of water-soluble saccharide and polyol (139). Bayer AG disclosed a foam prepared by using 1.5 to 50 parts of alkanolamine with water and 100 parts of polyisocyanate (138). Schaum Chemie disclosed foams prepared by using lower alkanols and alkylene diols (140). 

In rigid urethane foaming systems using the CO2 formed by the water-isocyanate reaction a balance of the relative rates between the urea... 

Arylazirines are carbonylated under very mild conditions (5 °C, 1 bar CO) in the presence of [Rh2(CO)4Cl2] to give arylvinyl isocyanates in high yield (equation 61). These products can be isolated or more conveniently converted to carbamates or ureas by reaction with alcohols or amines, respectively. 

Isocyanates also react with primary and secondary amine compounds. Tertiary amines cannot react with isocyanates because they do not contain active hydrogen atoms, but they are powerful catalysts for many other isocyanate reactions. Diamines are frequently used as chain extenders and curing agents in PU manufacture. The addition of a diamine to the reaction mixture increases the overall reactivity during polymerization. The reaction between an isocyanate group and an amine results in the formation of a urea bond. The polyurea segments present in the finished PU serve to increase the potential for both covalent and hydrogen bond crosslinks within the polymer. 

A new route to carbonyl di-isocyanate (Scheme 222)can be regarded as the formal reaction of urea with phosgene, since tri-n-butyltin isocyanate can be prepared from urea and the trialkyltin oxide, which is obtainable from the trialkyltin choride produced in the isocyanate reaction. Carbonyl di-isocyanate can be used to prepare triazines.Recent advances in the chemistry of... 

Metaphosphate species are often postulated to be found in reactions of phosphates. A high-pressure mass-spectrometric study of clustering of D2O and POJ suggests that the first two water molecules form simple adducts, but that addition of a third involves isomerization to a dihydrate of D2PO4 Metaphosphate is suggested as an intermediate in the reactions of phosphorus compounds containing P—OH bonds in molten urea. Isocyanic acid may be involved, as shown in Eqs. (33)-(36). Metaphosphate is not formed as an intermediate in the reactions... 

The formation of diastereomeric ureides from chloro-fomnate reagents has been used in quantitative assays and for the preparative separation of both secondary and tertiary amines. Quantitative assays for promethazine, a tertiary amine of the phenothiazine group of pharmaceuticals, have been developed. Maibaum [100] carried out a three-stage reaction in which the racemic promethazine was reacted with vinyl chloroformate and the carbamate was subsequently hydrolysed to the secondary amine. The enantiomeric ratio of the amine was then determined as the urea following reaction with a chiral isocyanate. In a later development, Witte et al. [lOI] directly derivatized the same compound with (— )-menthyl chloroformate (17) and separated the diastereomers by HPLC. (—)-Menthyl chloroformate has also been used to resolve the enantiomers of nomicotine as... 

The first reaction proceeds the most readily and at the lowest temperature. Each subsequent reaction occurs a little less readily and at a slightly higher temperature, but all proceed at normal atmospheric temperatures. Further reactions can proceed with the active hydrogens in the urea, urethane and amide products, but these are less important, except at paint stoving temperatures (100 °C and above). Catalysts, which speed reactions when added in small amounts, are frequently used for isocyanate reactions. 

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